soil composition to the north around Cypress Creek; most of the surface there consists of tan-colored sand with small amounts of gray clay. The north and northwestern regions of Houston and Harris County feature a slightly steeper slope than other parts of the city, with occasional escarpments caused by faulting or erosion. The sedimentary layers underneath Houston ultimately extend down some , with the oldest beds deposited during the Cretaceous. Between and below the surface is a layer of salt, the primary source of salt domes that dot the metropolitan area. Since salt is more buoyant than other sediments, it rises to the surface, creating domes and anticlines and causing subsidence due to its removal from its original strata. These structures manage to capture oil and gas as it percolates through the subsurface; Pierce Junction is a notable example of a heavily drilled salt dome oil field in Houston. Seismic activity The Houston region is generally earthquake-free, despite the presence of 86 mapped and historically active surface faults with an aggregate length of 149 miles (240 km). This includes the Long Point–Eureka Heights fault system which runs through the center of the city. No significant historically recorded earthquakes have occurred in Houston, but researchers do not discount the possibility of such quakes having occurred in the deeper past, or occurring in the future. Land subsidence has occurred across Greater Houston, primarily due to the pumping of water from subsurface aquifers (see Groundwater section). This subsidence may also be associated with slip along the faults; however, the slippage is slow and not considered an earthquake, in which stationary faults must slip suddenly enough to create seismic waves. The clay below the surface precludes the buildup of friction that produces ground shaking in earthquakes. As a result, faults generally move at a smooth rate in what is termed fault creep. The recently built Premium Outlet Mall and newly renovated US 290 have experienced the slippage of the Hockley Fault, which was redefined as being farther east than previously mapped. Surface water Rivers Houston, often popularly referred to as the Bayou City, is crossed by a number of slow-moving, swampy rivers, which are essential to draining the region's broad floodplains. The city was founded at the convergence of Buffalo Bayou and White Oak Bayou, a point today known as Allen's Landing. Buffalo Bayou is the longest and largest of the bayous which flow